Global S&T Development Trend Analysis Platform of Resources and Environment
The authors report near-atomic resolution structures of the R-type bacteriocin from Pseudomonas aeruginosa in the pre-contraction and post-contraction states, and these structures provide insight into the mechanism of action of molecular syringes.
R-type bacteriocins are minimal contractile nanomachines that hold promise as precision antibiotics(1-4). Each bactericidal complex uses a collar to bridge a hollow tube with a contractile sheath loaded in a metastable state by a baseplate scaffold(1,2). Fine-tuning of such nucleic acid-free protein machines for precision medicine calls for an atomic description of the entire complex and contraction mechanism, which is not available from baseplate structures of the (DNA-containing) T4 bacteriophage(5). Here we report the atomic model of the complete R2 pyocin in its pre-contraction and post-contraction states, each containing 384 subunits of 11 unique atomic models of 10 gene products. Comparison of these structures suggests the following sequence of events during pyocin contraction: tail fibres trigger lateral dissociation of baseplate triplexes
Despite being only one-atom thick, defect-free graphene is considered to be completely impermeable to all gases and liquids(1-10). This conclusion is based on theory(3-8) and supported by experiments(1,9,10) that could not detect gas permeation through micrometre-size membranes within a detection limit of 10(5) to 10(6) atoms per second. Here, using small monocrystalline containers tightly sealed with graphene, we show that defect-free graphene is impermeable with an accuracy of eight to nine orders of magnitude higher than in the previous experiments. We are capable of discerning (but did not observe) permeation of just a few helium atoms per hour, and this detection limit is also valid for all other gases tested (neon, nitrogen, oxygen, argon, krypton and xenon), except for hydrogen. Hydrogen shows noticeable permeation, even though its molecule is larger than helium and should experience a higher energy barrier. This puzzling observation is attributed to a two-stage process that involves dissociation of molecular hydrogen at catalytically active graphene ripples, followed by adsorbed atoms flipping to the other side of the graphene sheet with a relatively low activation energy of about 1.0 electronvolt, a value close to that previously reported for proton transport(11,12). Our work provides a key reference for the impermeability of two-dimensional materials and is important from a fundamental perspective and for their potential applications.
